High Efficiency CIGSS Thin Film based Solar Cells and Mini-modules

Abstract

The highest efficiency for Cu(Ga,In)Se₂ (CIGS) thin-film-based solar cells has been achieved with CdS buffer layers prepared by a solution growth method known as the chemical bath deposition (CBD). With the aim of developing Cd-free chalcopyrite-based-thin-film solar cells, we describe the basic concepts involved in the CBD technique. The recipes developed in our laboratory for the heterogeneous deposition for good-quality thin films of ZnO, ZnSe, ZnS are presented. In view of device optimization, the initial formation of chemical-bath-deposited ZnSe thin films on Cu(Ga,In)(S,Se)₂ (CIGSS) and the subsequent development of the CIGSS/ZnSe heterojunction were investigated by X-ray photoelectron spectroscopy (XPS). The good surface coverage was controlled by measuring changes in the valence-band electronic structure as well as change in the In_4d , Zn_3d core lines. From these measurements, the growth rate was found to be around 3.6 nm/min. The valence band and the conduction band-offsets E_V and E_C between the layers were determined to be 0.60 and 1.27 eV, respectively for the CIGSS/ZnSe interface. The energy band diagram is discussed in connection with the band-offsets determined from XPS data. Small and large area CIGSS based solar cells with a CIGSS/Zn(Se,OH)/ZnO structure are produced. A ZnSe thickness below 10 nm has been found to be optimum for achieving a homogeneous and compact buffer layer on CIGSS with an active area efficiency of 15.7%. Using Siemens Solar technology this structure is adapted to fabricate Cd-free CIGSS monolithic mini-modules with efficiencies up to 11.7%.